1. Field of the Invention
The present invention relates to an X-ray image taking device that records an X-ray image.
2. Related Background Art
With respect to X-ray image taking for medical diagnosis, a film screen system in which an intensifying screen and an X-ray photograph film are combined is often used. According to this method, because an X-ray transmitted through an object includes internal information of the object, the internal information is converted into visible light proportional to an intensity of the X-ray by the intensifying screen to photosensitive the X-ray film, thereby forming an X-ray image on the X-ray film.
In recent years, a digital image taking device has been widely used for a general image taking region such as a chest, in which an image is taken using the X-ray film up to now. Accordingly, it is possible to capture a digital image in the field of diagnostic image.
A high resolution digital X-ray detector has been already proposed. This X-ray detector is composed of a two-dimensional array using 3000 to 4000 detection elements represented by photo diodes or the like in the respective dimensions. Each of the detection elements produces an electrical signal corresponding to a brightness of a pixel of an X-ray image projected to the detector. The electrical signal from each of the detection elements is separately read and converted into a digital signal. After that, an image process, a storing operation, and a display operation are conducted.
FIG. 2 is a schematic view showing a sectional structure of an image taking unit of a digital X-ray image taking device. As shown in FIG. 2, the image taking unit is composed of a phosphor 501 and a photoelectric conversion element 502, which are used in combination. With respect to the photoelectric conversion element 502, for example, an amorphous silicon film (a-Si film) is used as a photoelectric conversion layer.
The amorphous silicon film is suitable on the following points. That is, the amorphous silicon film can be formed on a large-size sensor substrate such as a glass substrate. In addition, the amorphous silicon film can be used as a semiconductor material for a TFT serving as a switching element. The X-ray incident into the digital X-ray image taking device is converted into visible light by the phosphor 501. Then, a photo carrier is produced by light absorbed in the semiconductor layer of the photoelectric conversion element 502 and stored therein.
In the digital X-ray image taking device, in addition to an X-ray detecting unit for capturing an image, another X-ray detecting unit is provided for setting an image taking drive timing. The X-ray detecting unit for setting the image taking drive timing is used to measure a period from an irradiation instruction issued to an X-ray generator to an actual X-ray irradiation (pre-delay period) or a period from the completion of the issue of the X-ray irradiation instruction to the completion of the actual X-ray irradiation (post-delay period).
These periods are varied according to a system structure of the X-ray image taking device and each X-ray generator. Therefore, it is necessary to measure the periods at the time of setting each system and to set drive timings.
It is necessary to set the pre-delay period in the case where the digital X-ray image taking device has a movable grid. Predetermined linear movement of the movable grid is required during the capture of an X-ray image. Therefore, a timing for starting the grid movement operation is adjusted in accordance with a set value of the pre-delay period.
It is necessary to set the post-delay period in order to capture a preferable X-ray image by reducing a crosstalk among sensor pixels. It is proposed in Japanese Patent Application Laid-Open No. 2002-055164 that image reading from the sensor pixels is started after a lapse of a predetermined time from the completion of the actual X-ray irradiation, so that a preferable X-ray image in which the crosstalk among the sensor pixels is reduced can be captured. A timing for starting image reading from the completion of the transmission of an X-ray irradiation instruction signal is adjusted in accordance with a set value of the post-delay period.
The X-ray detector for setting the image taking drive timing is located in the front or the rear of a sensor near the center of the image taking unit. This is because the X-ray detector has also a function as an X-ray monitor that detects a state of the X-ray irradiation each time image taking is performed.
However, in the case where the X-ray detector is located near the center of the image taking unit, a case structure of the image taking unit is complicated, which leads to an increased cost. In addition, a defect in which the X-ray detector and its attachment mechanism are reflected on an X-ray image is caused due to an adverse influence of a back-scattered radiation.
Up to now, in order to capture an image with a preferable image quality, it is necessary to minimize a charge accumulation time of the sensor. Therefore, it is required that the completion of the X-ray irradiation is detected each time image taking is performed. However, in recent years, a sensor capable of capturing a preferable image with a low noise even if a charge accumulation time which is not minimized is set has been developed. In particular, in the case of an X-ray image taking device in which a fixed charge accumulation time drive is employed, it is not essential to detect the completion of the X-ray irradiation each time image taking is performed.